1. Field of the Invention
The present invention relates to displays, particularly electrophoretic displays (electronic papers), and to new structures useful in such displays.
2. Description of the Related Art
An electrophoretic display (electronic paper) which utilizes the phenomenon of electrophoresis to achieve contrast is an electronic-indicated display using movement of a charged particle in an applied electric field. It is possible to apply the electrophoretic display to displays such as electronic books, electronic dailies, electronic magazines, electronic publications, and information displaying media of mobile communication devices.
Electrophoresis refers to movement of charged particles in an applied electric field. When an electrophoresis occurs in a fluid, the charged particles move with a velocity determined primarily by the viscous drag of the particles, their charge, the dielectric properties of the fluid, and the magnitude of the applied electric field.
An electrophoretic display utilizes charged particles of one color suspended in a dielectric fluid medium of a different color to achieve contrast. In other words, when the electrodes are operated to apply an electric field across the medium, charged particles having opposite sign to each other migrate toward the electrode of opposite sign, respectively. The result is a visually observable color change.
Useful electrophoretic displays are bistable, their state persists even after the activating electric field is removed. Unfortunately, the stability of current electrophoretic displays is limited. Although flocculation or settling of particles can be avoided by matching the density of the particles with that of the fluid medium, long-term particle agglomeration remains a problem. Moreover, the problem becomes even worse when two particles of having different color and opposite sign to each other migrate by the electrophoresis phenomenon, which deteriorates the displays.
To solve the problems described above, E-Ink Corporation in the U.S.A has suggested electrophoretic displays are based on microcapsules each having therein an electrophoretic elements of a dielectric fluid and a suspension of particles that visually contrast with the dielectric fluid and also exhibit surface charges. The successful construction of an electrophoretic display requires the proper interaction of several different types of materials and processes. Materials such as a polymeric binder, a capsule membrane, and the electrophoretic particles and fluid must all be chemically compatible.
FIGS. 1A and 1B are sections of a conventional electrophoretic display.
FIG. 1A illustrates an encapsulated electrophoretic display when an electric field is not applied. In the electrophoretic display, a lower electrode 13 and a lower electrode protection layer 15 are formed on an under layer 11 that may be transparent or opaque. A microcapsule 17 is formed on the lower electrode protection layer 15. The microcapsule 17 comprises a transparent fluid 19, a white particle of positive charge 21, and a black particle of negative charge 23. The lower electrode protection layer 15 has functions of protecting the lower electrode 13 and separating the lower electrode 13 from the microcapsule 17. An upper electrode 25 is located on the microcapsule 17. A transparent upper layer 27 is formed on the upper electrode 25.
FIG. 1B illustrates an encapsulated electrophoretic display when an electric field is applied. When an electric field is applied between a lower electrode 13 and an upper electrode 25, the particles of positive charge 21 and negative charge 23 within the microcapsule 17 migrate toward an electrode of opposite sign. The migration of the charged particles causes a visually observable color change.
However, in the prior art, when two charged particles of different colors exist within the microcapsule 17, the electrophretic display has to keep the same specific gravities between two charged particles of different colors and a dielectric fluid, and also needs a chemical treatment to prevent between two charged particles from agglomerating. Such a chemical treatment is very difficult to handle since the difference in the specific gravities of the charged particles and the dielectric fluid causes floating or sediment of the charged particles in a specific time or the agglomeration or clustering of the charged particles, and eventually leads to a deterioration of a display.
Another problem of conventional electrophoretic display having two charged particles is the difference in a size and a charge quantity of two charged particles causes the difference in the response (switching) time between two charged particles.